The aim of the present study was to evaluate sulfide production in bioreactors containing residual heavy oil as the electron donor and carbon source to more closely mimic souring and its control with nitrate in an actual oil field. Specifically, we compared limiting (4 mM) and excess (100 mM) concentrations of nitrate injected either as pulses or continuously. Interestingly, the data indicate that acetate is also a major intermediate in the sulfidogenic metabolism of oil.